Melting Glaciers Liberate Ancient Microbes

Editor's Note: This article is an extended version of "Bugs in the Ice Sheet" from the May 2012 Issue of Scientific American.

BOZEMAN, Mont.—Locked in frozen vaults on Antarctica and Greenland, a lost world of ancient creatures awaits another chance at life. Like a time-capsule from the distant past, the polar ice sheets offer a glimpse of tiny organisms that may have been trapped there longer than modern humans have walked the planet, biding their time until conditions change and set them free again.

With that ice melting at an alarming rate, those conditions could soon be at hand. Masses of bacteria and other microbes – some of which the world hasn't seen since the Middle Pleistocene, a previous period of major climate change about 750,000 years ago – will make their way back into the environment.

Once thought to be too harsh and inhospitable to support any living thing, the ice sheets are now known to be a gigantic reservoir of microbial life. Altogether, the biomass of microbial cells in and beneath the ice sheet may amount to more than 1,000 times that of all the humans on Earth.

Internment in the ice amounts to an evolutionary strategy for microorganisms: preserving genetic blueprints by storing them in deep–freeze for a future re-entry, said John Priscu, a Montana State University professor and pioneer in the study of Antarctic microbiology.

"It's a way of recycling genomes," he said. "You put something on the surface of the ice and a million years later it comes back out."

'Storehouse for genes'
Priscu has spent the past 28 Austral summers on the southernmost continent, studying what he calls "the bugs in the ice sheet." Antarctica has the oldest ice on Earth; parts of its glacial landscape date back about a million years, and some pockets are believed to be up to 8 million years old. "There's a lot of history in that ice sheet," said Priscu.

Much of that history appears to still be alive. Priscu has found living bacteria in cores of 420,000-year-old ice and gotten them to grow in his laboratory. Other researchers report bringing far older bacteria back to life.

The ice allows microbes to enjoy a sort of immortality, preserving ancient genetic material and allowing creatures that have long disappeared from the planet to someday return. "That's what's interesting about the ice – it can serve as a storehouse for those genes," said Jonathan Klassen, an evolutionary biologist and postdoctoral research fellow at the University of Wisconsin, Madison. "Things that went extinct have the possibility of coming back."

Could this be Jurassic Park on Ice? Not likely, scientists say. The only things able to survive in these cold, dark, crushed quarters with little to eat or drink are microscopic organisms, and most of what has been found appears related to microbes from other cold and icy environments.

Still, with heat-trapping greenhouse gases warming the polar regions much faster than the rest of the planet today, investigators have many questions about the bugs in the ice sheet. Researchers are trying to determine how these organisms can survive such a brutal habitat, some seeming to sit in what resembles a state of suspended animation for millennia. The findings could point the way for the discovery of life in other extreme climates, such as frozen planets and moons.

Methane emissions
The more immediate concerns sit here on Earth. Cells and carbon dumped out of melting glaciers could turn into huge piles of decomposing organic matter – compost – that generate carbon dioxide and methane as they decay, a potentially significant source of greenhouse gas emissions that climate researchers have yet to consider.

And scientists see evidence that the microbes are evolving inside the ice sheets, exchanging DNA and gaining new traits. While these cold-loving organisms appear to pose little threat to warm-blooded creatures, they could force out existing microbial populations, with unknown consequences. [See sidebar: Warming climate sets evolution within ice to high]

The frozen "bacteriasicles," as Louisiana State University microbiologist Brent Christner describes them, can emerge from the ice after hundreds of thousands of years poised to grow and divide when favorable conditions arise. Christner, an associate professor in the Department of Biological Sciences, has revived bacteria encased in 750,000-year-old ice.

"When we look in the oldest ice we can get our hands on, we still find there are cells living," he said.

It's a big deal, Christner added, because researchers don't understand how an organism can "sit for 750,000 years in some state of suspended animation like when Han Solo was put in carbonite."

Around since the Stone Age
It turns out that being immobilized in ice is actually a good way for microbes to preserve themselves. Inside those glaciers, they appear to be existing in a minimally-active state, maintaining their DNA and somehow repairing the damage incurred over time from radiation, oxidation and other harmful forces. They may be able to remain "frozen" like this almost indefinitely, according to Christner.

Being around since the Stone Age does take its toll on a body, however. The older the ice, Christner finds, the longer it takes the organism to revive.

The bugs in the ice are giving scientists an exciting opportunity to examine ancient genomes and learn about Earth's past climates from the organisms living in previous times of warming and cooling. "You kind of think of ice as a museum back in time, this window back into your past," said Christine Foreman, an associate research professor of microbial ecology at Montana State University who is examining microbes found in Antarctic ice deposited up to 60,000 years ago.

For Priscu, the fundamental question, at least in terms of paleoclimate, is whether different bacteria communities exist during glacial periods versus interglacial periods. "If we take our current warm period now, are the bacteria different than they were a million years ago?"

Priscu hopes to answer that question next winter, when he and an American research team drill into an ice stream below the West Antarctic Ice Sheet that may harbor descendants of microbes living on the continent when it began to freeze over more than 20 million years ago.

That effort is one of three underway to drill into long-buried Antarctic waterways and bring organisms sequestered under the ice into the light of modern day: In February, a Russian team pierced more than two miles of ice in remote east Antarctica to reach Lake Vostok, a body of water the size of Lake Erie that has been sealed off for 25 million years. And British scientists are drilling into a subglacial lake more than a mile below the continent's western surface.

Prehistoric pestilence
The notion of digging so deep for life raises a darker question: Could some prehistoric pestilence be lurking in that ice waiting for a chance to come back and terrorize a defenseless population? An Amundsen Strain, perhaps, named after Norwegian explorer Roald Amundsen, who led the first expedition to reach the South Pole 100 years ago.

''There are some possible adaptations by surviving in ice that could be advantageous to these pathogens," said Scott Rogers, an evolutionary biologist and professor in the Department of Biological Sciences at Bowling Green State University in Ohio. For instance, a virus becomes unable to cause infections when its host population builds up antibodies to it. "So if [viruses] can survive in the ice, they could come back at a time when the population is naïve again and hasn't been exposed to that particular genotype," he said.

Rogers has found a living 140,000-year-old plant virus in glacial ice from Greenland. He speculates that it could be possible for some hardy viruses, like the one that causes polio, to survive in ice and come back to infect susceptible humans. However, he and other scientists think there's little possibility that melting glaciers will unleash virulent viruses that trigger pandemics like the one depicted in the recent Hollywood horror film, Contagion. Human and animal pathogens have evolved to live in a warm, cozy place and would have a hard time surviving the extremely harsh conditions in ice. Viruses tend to be especially fragile.

"The chances aren't zero," said Rogers, "but they're very close to zero."

More likely is prospect that thawing ice sheets will allow ancient microbial genes to mix with modern ones, flooding the oceans with never-before-seen types of organisms. Rogers believes this is already taking place. "What we think is happening is that things are melting out all the time and you're getting mixing of these old and new genotypes," he said.

The biggest effect of these newly liberated – and potentially newly remade –microbes will likely be seen in the oceans, Christner said. Earth's glaciers and sub-glacial sediments contain more microbial cells and carbon than all the lakes and rivers on the surface of the planet – a huge load of organic matter that, if thawed, would end up in the sea, he said.

"Potential climate impacts are more in the direction of introducing nutrients into marine systems," Christner said. One scenario: The nutrients trigger growth bursts of bacteria that would then use up all the oxygen in the water, destroying fish habitats and exacerbating ocean dead zones that are already occurring from other causes.

"We have always thought about the ocean as being this sink that can handle everything," said Montana State's Foreman. "But we know now that's not true. There are going to be tipping points where things are going to happen… But we don't know what point that's going to be."

Enormous compost piles
Thawing glaciers could also churn out enormous compost piles of decaying biomass. All the carbon from organic matter in and under the ice sheets, if converted to carbon dioxide, would equal a decade's worth of emissions from vehicles on roads worldwide, scientists estimate. Of course, not all of the carbon would convert directly to greenhouse gases. But any release would add to the huge amount expected from thawing permafrost. "This is a big pool of carbon to be considered," Priscu said. "The carbon cycle is so important right now, especially in terms of climate change, that we really should look at this."

So while these defrosted bugs may not be appearing in a sci-fi thriller at your local movie house any time soon, they do present another worry for the future of the polar regions, scientists say. And when considered alongside rising seas, shifting wildlife habitats, a diminishing planetary albedo and other manifestations of a changing climate, the biggest impact of this newly liberated biological stockpile could be its ability to tip the planet's shaky equilibrium.

Priscu likens the state of today's climate to a light switch being tripped.

"If you hold that light switch right there before it flips, the lights begin to flicker. I think that's what we're seeing now," he said. "We're pushing it and it's becoming more variable, and pretty soon it'll pass a threshold and reach a new state. Whether or not it can go back to a previous state, we don't know… We may end up not ever being able to go back."